Despite all of the security it offers, quantum cryptology also has a few fundamental flaws. Chief among these flaws is the length under which the system will work: It’s too short.
The original quantum cryptography system, built in 1989 by Charles Bennett, Gilles Brassard and John Smolin, sent a key over a distance of 36 centimeters. Since then, newer models have reached a distance of 150 kilometers (about 93 miles). But this is still far short of the distance requirements needed to transmit information with modern computer and telecommunication systems.
The reason why the length of quantum cryptology capability is so short is because of interference. A photon’s spin can be changed when it bounces off other particles, and so when it's received, it may no longer be polarized the way it was originally intended to be. As the distance a photon must travel to carry its binary message is increased, so, too, is the chance that it will meet other particles and be influenced by them.
One group of Austrian researchers may have solved this problem. This team used what Albert Einstein called “spooky action at a distance.” This observation of quantum physics is based on the entanglement of photons. At the quantum level, photons can come to depend on one another after undergoing some particle reactions, and their states become entangled. This entanglement doesn’t mean that the two photons are physically connected, but they become connected in a way that physicists still don't understand. In entangled pairs, each photon has the opposite spin of the other. If the spin of one is measured, the spin of the other can be deduced. What’s strange (or “spooky”) about the entangled pairs is that they remain entangled, even when they’re separated at a distance.
The Austrian team put a photon from an entangled pair at each end of a fiber optic cable. When one photon was measured in one polarization, its entangled counterpart took the opposite polarization, meaning the polarization the other photon would take could be predicted. It transmitted its information to its entangled partner. This could solve the distance problem of quantum cryptography, since there is now a method to help predict the actions of entangled photons.
Even though it’s existed just a few years so far, quantum cryptography may have already been cracked. A group of researchers from Massachusetts Institute of Technology took advantage of another property of entanglement. In this form, two states of a single photon become related, rather than the properties of two separate photons. By entangling the photons the team intercepted, they were able to measure one property of the photon and make an educated guess of what the measurement of another property - like its spin - would be. By not measuring the photon’s spin, they were able to identify its direction without affecting it. So the photon traveled down the line to its intended recipient none the wiser.
The MIT researchers admit that their eavesdropping method may not hold up to other systems, but that with a little more research, it could be perfected. Hopefully, quantum cryptology will be able to stay one step ahead as decoding methods continue to advance.
Assignments
1. Translate the sentences from the texts into Russian in writing paying attention to the underlined words and phrases:
1. By harnessing the unpredictable nature of matter at the quantum level, physicists have figured out a way to exchange information on secret keys.
2. After the entire transmission, Bob and Alice have a non-encrypted discussion about the transmission.
3. Since Bob isn't saying what his measurements are -- only the type of filter he used -- a third party listening in on their conversation can't determine what the actual photon sequence is.
4. One of the great challenges of cryptology is to keep unwanted parties – or eavesdroppers -- from learning of sensitive information.
5. In modern cryptology, Eve (E – an eavesdropper) can passively intercept Alice and Bob's encrypted message -- she can get her hands on the encrypted messag e and work to decode it without Bob and Alice knowing she has their message.
6. After Eve has measured the photons by randomly selecting filters to determine their spin, she will pass them down the line to Bob.
7. If discrepancies are found, they should occur in 50 percent of the parity checks. Since Eve will have altered about 25 percent of the photons through her measurements, Bob and Alice can reduce the likelihood that Eve has the remaining correct information down to a one-in-a-million chanc e by conducting 20 parity checks.
8. As the distance a photon must travel to carry its binary message is increased, so, too, is the chance that it will meet other particles and be influenced by them.
9. At the quantum level, photons can come to depend on one another after undergoing some particle reactions, and their states become entangled.
2. Answer the following questions:
1. How can quantum physics help users to exchange information securely?
2. How does a photon become a key?
3. Can the users communicate openly using photons for encryption?
4. How can quantum cryptology safeguard against passive interception?
5. How does the parity check work?
6. What are the main flaws of quantum cryptology?
7. Is it possible to increase the quantum cryptology capability?
3. Translate into English:
Наибольшее практическое применение КК находит сегодня в сфере защиты информации, передаваемой по волоконно-оптическим линиям связи (ВОЛС). Это объясняется тем, что оптические волокна ВОЛС позволяют обеспечить передачу фотонов на большие расстояния с минимальными искажениями. В качестве источников фотонов применяются лазерные диоды передающих модулей ВОЛС; далее происходит существенное ослабление мощности светового сигнала - до уровня, когда среднее число фотонов на один импульс становится много меньше единицы. Системы передачи информации по ВОЛС, в приемном модуле которых применяются лавинные фотодиоды в режиме счета фотонов, называются квантовыми оптическими каналами связи (КОКС).
Понятно, что вследствие малой энергетики сигналов скорости передачи информации в КОКС по сравнению с возможностями современных ВОЛС не слишком высоки (от килобит до мегабит в секунду, в зависимости от реализации). Поэтому в большинстве случаев квантовые криптографические системы (ККС) применяются для распределения ключей, которые затем используются средствами шифрования высокоскоростного потока данных. Необходимо отметить, что квантово-криптографическое оборудование пока серийно не выпускается. Однако по мере совершенствования и удешевления применяемой элементной базы можно ожидать появления ККС на рынке телекоммуникаций в качестве, например, дополнительной услуги при построении корпоративных волоконно-оптических сетей.
